Molecules of Elements:-
The molecule of an element are constitute by the same type of atoms. Such Molecules are called Homoatomic Molecules. For example,a molecule of oxygen consists of two atoms of oxygen (O2) and if 3 atoms of oxygen combine into a molecule ,we get ozone (O3).
The number of atoms present in a molecule of an element is called it's atomicity.
Molecules of argon (Ar),helium(He)etc. are made up of only one atom of that element and hence they are called monoatomic.
Oxygen, hydrogen, nitrogen and chlorine are diatomic molecule.
Some elements have 3,4or more atoms of that element and hence are called triatomic, tetra-atomic and polyatomic respectively.
Molecules of Compounds:-
Atoms of different elements join together in definite proportions to form molecules of Compounds. Such Molecules are called Hetroatomic Molecules. For example two atoms of hydrogen and one atom of oxygen combine to make one molecule of water.